Applications – Chassis & Suspension – Subframes
Applications – Chassis & Suspension – SubframesTable of Contents1 Axle subframes . 21.1 Introduction . 21.2 Rear axle subframes . 41.2.1 Rear subframe as a one-piece casting. 51.2.2 Rear axle subframe in aluminium sheet design . 71.2.3 Rear axle subframes assembled from different aluminium products. 91.3 Front axle subframes . 14Version 2011 European Aluminium Association (auto@eaa.be)1
1 Axle subframes1.1 IntroductionSubframes are structural modules which are designed to carry specific automotivecomponents such as the engine or the axle and suspension. The purpose of using asubframe in an automobile is to distribute high local loads over a wider area of the bodystructure (most relevant in thin-walled monocoque body designs) and to isolate vibration andharshness from the rest of the body. The subframes are bolted or welded to the vehicle body.Bolted subframes are sometimes equipped with rubber bushings or springs to dampen noiseand vibrations. An additional benefit is that subframes can be separately assembled andintegrated into the vehicle on an automated assembly line when required.As a natural development from a car with a full chassis, separate front and rear axlesubframes are used in modern vehicle designs to reduce the overall weight and cost. Axlesubframes can have various forms and fulfill different functions:oosubframes for rear and front axlesperimeter frames which carry both the axle and the engine (possibly including thetransmission and the full suspension).Simple axle subframes usually carry the axle, the lower control arms and, in case of the frontaxle, the steering rack. Subframes which also support the engine and possibly othercomponents (e.g. transmission) would be particularly useful on front wheel drive cars. Suchmore complex, but also more expensive designs would result in better road isolation and lessharshness since these components are not anymore directly connected to the main bodystructure. However, no specific examples can be shown.BMW 5 series model with aluminium front and rear axle subframesSource: BMWVersion 2011 European Aluminium Association (auto@eaa.be)2
Axle subframes must be stable to ensure excellent road contact, but also light to guaranteehigh occupant comfort regardless of surface unevenness. Therefore, they are most interestingcomponents for the application of aluminium:oooooProperly designed aluminium modules show the required strength and stiffness foraxle subframes.In addition to the general lightweighting benefit, lightweighting of unsprung massesreduces the vibration forces and offers a smoother ride.The structural functionalities of axle subframes ask for fairly complex geometricalshapes and the need to integrate different attachment points. Consequently, thepotential of the aluminium extrusion technology and the various high qualityaluminium casting methods for the integration of additional functions into a structuralpart can be fully exploited.Depending on the production volume, the fabrication cost of aluminium subframescan be reduced by the application of properly designed high quality aluminiumcastings (reduction of assembly cost by part integration). Furthermore, theelimination/reduction of assembly joints improves the overall performance of thesubframe.The possibility to assemble the aluminium subframe separately from the rest of thevehicle facilitates its integration into a steel or mixed material body.Therefore, many different lightweight subframe designs have been developed using thevarious aluminium product forms, i.e. sheets, extrusions and castings. In the following, somealuminium axle subframes currently in series application will be presented. But also a varietyof other aluminium designs which have been used in the past or have been developed up tothe prototype scale are shown.The market share of aluminium in this application is today still small. With the developmentand market introduction of dual phase steel grades and other advanced high strength steels,some promising aluminium projects for axle subframes and engine cradles have beenstopped or delayed. Nevertheless, there is still significant growth potential for aluminium inaxle subframes and engine cradles. The increasing demand for lightweighting, combined withthe benefits of reduced unsprung masses, will clearly serve as a driver for future marketgrowth.Version 2011 European Aluminium Association (auto@eaa.be)3
1.2 Rear axle subframesAluminium rear axle subframes are particularly prominent for rear wheel drive cars with highdemands on driving dynamics and comfort. The axle must hold the wheels on the road inorder to ensure constant and even traction of the drive wheels. Thus it is important to create astructural module of especially low weight and high stiffness. In addition, severe packagerestrictions and the quality requirements of a safety functional chassis part represent furthercriteria to be considered. The complexity and size of the subframe combined with the highlevel of requirements represent a new dimension in the design of aluminium chassis parts forseries application.Rear axle of the Porsche PanameraSource: PorscheVersion 2011 European Aluminium Association (auto@eaa.be)4
1.2.1 Rear subframe as a one-piece castingAn obvious solution for a lightweight aluminium rear axle subframe is a one-piece hollowcasting. An early example (start of production 1998) is shown below. The relatively largecasting (length 700 mm, width 1200 mm, height 250 mm) and a finished part weight of 16.7kg has been produced by the Vacuum Riserless Casting (VRC) / Pressure Riserless Casting(PRC) technology developed by Alcoa. The wall thickness of the cast subframe variesbetween 5 and 25 mm, the A356 alloy is used in the as-cast state. The substitution of theoriginal steel stamping by a cast aluminium subframe enabled a weight reduction of nearly40%.Rear axle subframe as a one-piece hollow casting produced by the VRC/PRCtechnologySource: AlcoaA more recent example, produced by Farsund Aluminium Casting AS using the low pressuredie casting process (VRC/PRC), is the Porsche Panamera rear subframe. The rather smallnumber of annual production units requires a production process which is economic for lowervolumes. In order to limit cost, but maintain light weight, the subframe was designed as a onepiece casting with a high degree of functional integration. The selection of a casting techniqueoffering maximum part performance enabled not only a complete hollow design with thinwalls, but also to meet the high strength and stiffness demands of a highly stressed chassispart. With dimensions of 1200 by 710 by 335 mm, a nominal wall thickness of 3.9 mm and aweight of only 16 kg, the subframe component consolidates about 30 individual parts of theusual sheet panel design into one single casting.Version 2011 European Aluminium Association (auto@eaa.be)5
Rear axle subframe of the Porsche Panamera, produced as a one-piece hollow castingby low pressure die casting (VRC/PRC)Source: PorscheBut also other high quality casting methods such as aluminium sand casting can be used forthe production of rear axle subframes.Prototype of a hollow aluminium rear axle subframe produced by sand casting in thealloy AlSi7Mg (final weight 17.1 kg)Source: GF AutomotiveVersion 2011 European Aluminium Association (auto@eaa.be)6
1.2.2 Rear axle subframe in aluminium sheet designNon heat-treatable AlMgMn alloys are applied in large quantities as hot and cold rolled sheetsdue to their good formability which can always be re-established by interannealing duringcomplex forming operations. They are highly suitable for the assembly of relatively complexchassis parts such as axle subframes because of their formability and strength, weldabilityand the fact that there is no need for quenching for age hardening (which would bedetrimental for the required consistent geometrical tolerances).A well established AlMg alloy for high strength and excellent formability is EN AW-5182(AA5182). However, in 5xxx alloys containing 3% Mg, the precipitation of β - Mg 5 Al 8particles at grain boundaries during long term exposure at temperatures 80 C can result ina susceptibility to intergranular corrosion cracking. Thus, AlMg alloys with a higher Mg contentmust be used with caution in applications where exposure to elevated temperature cannot beexcluded. The material of choice for chassis parts are therefore the medium strength alloys ofthe type AlMg3Mn (EN AW-5754) and AlMg3.5Mn (EN AW-5454).Aluminium rear axles produced from AlMgMn aluminium alloy sheetsSource: Hydro Aluminium Rolled ProductsThe rear axle subframe of the BMW 5 series models is a compact structural moduleconsisting of hydroformed tubes and deep-drawn sheets which is assembled by MIG welding.With a weight of approx. 11.5 kg, it offers a weight reduction of about 40% compared to asteel solution. The hollow supporting tubes guarantee a high flexural and torsional stiffnesswhich minimises the negative impact of the lower elastic modulus of aluminium on thevehicle's driving dynamics. Longitudinally seam-welded tubes (produced by HF welding)make up approx. 70 % of the subframe with the remaining 30 % comprised of cold-rolled anddeep drawn sheets. The complex shape of the tubes in the required narrow tolerances isachieved by 3D-bending, suitable pre-forming and final hydroforming. The bearing bushes forfixing the plastic bearings are made from extrusions (cut-to-length) while the brackets forattaching the control arms are sheet-metal constructions. The wall thickness of the tubes andthe sheet stampings is 3.5 – 4 mm, of the extrusions 3 – 6 mm. The EN AW-5454(AlMg3.5Mn) alloy is used for the tubes and the sheet stampings, the extrusions are made ofthe alloy EN AW-6060 (Al MgSi0.5).Version 2011 European Aluminium Association (auto@eaa.be)7
Rp0,2RmElongation[MPa][MPa][%]Tubes (0/H111): 105 240A5 18Sheets (H24): 190 270A5 8Extrusions (T4): 60 120A10 13Mechanical properties:Rear axle subframe for BMW 5 series modelsSource: Hydro Aluminium Rolled ProductsPre-fabricated tube parts for the BMW 5 series rear axle subframeSource: Hydro Aluminium Rolled ProductsVersion 2011 European Aluminium Association (auto@eaa.be)8
Another fabrication method has been chosen for the aluminium subframe of the Mercedes Sclass. In this case, the rear axle subframe is a MIG welded assembly whose components aremainly made from deep-drawn aluminium sheet and a few cut-to-length extrusions. Therespective longitudinal members and rear cross member consist of hollow structures whichare manufactured by fitting half-shells together; the front cross member and brackets are ofopen sheet-metal design. The bearing bushes for fixing the plastic bearings are formed duringthe deep-drawing process. With a weight of 12.5 kg, the achieved weight reduction comparedto a steel solution is also approx. 40%. Hot-rolled sheets of the alloy with a thickness of 2.5 –3.5 mm of the alloy EN AW-5754 (AlMg3Mn) and extrusions with 3.5 mm wall thickness of thealloy EN AW-6060 (AlMgSi0.5) are applied.Rp0,2RmElongation[MPa][MPa][%]Sheets (H0/111): 85 215A5 17Extrusions (T4): 60 120A10 13Mechanical properties:Rear axle subframe of the Mercedes S classSource: Hydro Aluminium Rolled Products1.2.3 Rear axle subframes assembled from different aluminiumproductsWhereas one-piece castings are primarily of interest for relatively small production volumes,the assembly of rear axle subframes from sheet stampings is rather a cost-effective solutionfor large series. Assemblies using different aluminium product forms (sheet stampings, preformed and/or machined extrusions and castings) may well be considered for mediumproduction volumes.The aluminium rear axle subframe of the BMW 7-series is a combination of hydroformedlongitudinally welded tubes and cast components with a total weight of only 14.1 kg. Thesubframe is joined by MIG welding.Version 2011 European Aluminium Association (auto@eaa.be)9
Aluminium rear axle subframe of the BMW 7 seriesSource: Hydro Aluminium Rolled ProductsA combination of a sheet stamping with a casting was also used for a prototype rear axlesubframe which was designed for an upper class passenger car:-Dimensions: (L / W / H in mm):Finished part weight (in kg):1130 / 675 / 26513Aluminium rear axle subframe prototype consisting of a sheet stamping and a singlecasting (below, bottom view)Source: Hydro Aluminium Rolled ProductsVersion 2011 European Aluminium Association (auto@eaa.be)10
The estimated weight benefit compared to the existing steel solution, which includes severalstamped parts joined by numerous arc welded joints, was approx. 35 %. The choice of thealuminium solution mixing a cast part and a stamped sheet was guided by the wish to limit thenumber of joints. The proposed design allows to reduce the assembly cost and to balance therelatively poor fatigue behaviour of welded aluminium joints. The selection of a single castpart offers the possibility to integrate additional functions and to avoid several attachments.The weld has been placed near to the neutral axis of the part in order to reduce stress inwelded area.The thickness of the applied aluminium sheet was 3 mm, the alloy EN AW-6061 – T4. Thewall thickness of the casting varied between 4 and 7 mm, alloy Calypso 61S (AlSi10Mg) – F.Low pressure die casting was foreseen for series production, the prototypes were made bysand casting. The casting alloy AlSi10Mg has been preferred over AlSi7Mg since it is moreappropriate for large and thin parts. An AlMgSi (6xxx series) alloy has been chosen forwelding compatibility with the casting alloy AlSi10Mg. It is used without any thermal treatmentin order to avoid distortions that could occur during quenching and to avoid loss of mechanicalproperties during welding with the filler wire AA 4043.An assembled rear axle subframe consisting of extruded profiles (wall thickness 5 – 10 mm)and thixo-cast parts has been used in the Alfa Romeo Spider:-Dimensions: (L / W / H in mm):Finished part weight (in kg):1270 / 630 / 28017.8The straight extruded sections of the alloy EN AW-6061 in the T6 temper were only slightlymachined. The thixo-cast component (Althix 67S1 (AlSi7Mg0.6) – T5) was produced byStampal SPA. Thixo-casting was selected to manufacture the shaped component because itprovides near net shape parts reducing the final machining needs and thus minimisation ofcost. The components were joined by MIG welding.Rear axle subframe made from straight extrusions and thixo-cast components,produced for the Alfa Romeo SpiderSource: ConstelliumVersion 2011 European Aluminium Association (auto@eaa.be)11
Thixo-cast arm (bottom view)Source: ConstelliumAlso the rear axle subframe of the Citroën C5 was an assembly of aluminium extrusions andcastings:-Dimensions: (L / W / H in mm):Finished part weight (in kg):1260 / 570 / 22011.7.Compared to the former solution, a steel tube and two cast iron parts joined by machining andforce fitting, the aluminium solution offered a weight reduction of 45%.Rear axle subframe of the Citroën C5Source: ConstelliumThe wall thickness of the extruded section was 2.5 mm (alloy EN AW-6005A – T6),subsequent fabrication included cutting and piercing. The cast components with wallthicknesses between 4 and 15 mm were produced by permanent mould gravity casting usingthe alloy Calypso 67B (AlSi7Mg) – T6. The parts were then joined by press fitting and MIGwelding. The joint is a butt weld on a natural support provided by the cast part (see photoVersion 2011 European Aluminium Association (auto@eaa.be)12
below). The assembly cost could be reduced compared to the earlier solution due to theelimination of the machining step which was necessary for force fitting of the steel tube ontothe cast iron parts. The aluminium extrusion is just press fitted onto the cast parts and thenwelded.Detail of the rear axle subframe of the Citroën C5Source: ConstelliumA similar solution consisting of two castings and an extrusion was chosen for the rearsuspension crossmember of the Chrysler Concorde/Dodge Intrepid. Compared to the formersteel solution, the aluminium design with a finished part weight of 5.8 kg offers a 30 – 35%lower mass, but doubles the lateral stiffness and even triples the torsional stiffness.Rear suspension cross member of the Chrysler Concorde/Dodge IntrepidSource: AlcanVersion 2011 European Aluminium Association (auto@eaa.be)13
1.3 Front axle subframesLightweighting is also a strong driver for the application of aluminium in the front axlesubframe. An excellent example is the BMW double-joint spring-strut front axle which isconstructed of aluminium. The use of aluminium leads to a weight reduction of 30% comparedto a similar axle made of steel, minimising the unsprung mass of the vehicle. The applicationof aluminium delivers immediate results in the responsiveness of the suspension and steeringsystems. The lower the weight that has to be moved, the easier it is to control.At the same time, the aluminium structure is so stable that the front wheels always haveoptimum traction on the road surface, thanks to the highly rigid diagonal front axle subframewhich also carries the steering gears, track control arms and push bars, as well as the anti-rollbar.Aluminium front axleSource: BMWA different example is the front-axle subframe which was used for the lightweight version ofthe Volkswagen Lupo (“3 litre Lupo”). In this case, a very simple design consisting of a onepiece, folded sheet-metal structure has been used. Following a multistage deep-drawing andstamping process, the aluminium sheet is folded to give a hollow transverse member. Theoverlapping sheet ends are joined together by MIG welding. Collateral squeeze-castaluminium brackets and forged control arms are attached to the transverse member.Version 2011 European Aluminium Association (auto@eaa.be)14
Front axle subframe of the Volkswagen “3 litre” LupoSource: Hydro Aluminium Rolled ProductsWith a final weight of approx. 2.6 kg, the resulting weight reduction compared to a steelsolution was approx. 45%. A hot rolled aluminium sheet of the alloy EN AW-5454 (AlMg3Mn)with a thickness of 3.5 mm was used. The mechanical properties are:Mechanical properties:Sheets (H0/111):Rp0,2RmElongation[MPa][MPa][%] 85 215A5 17Intermediate fabrication stepsSource: Hydro Aluminium Rolled ProductsVersion 2011 European Aluminium Association (auto@eaa.be)15
The fabrication of the subframe includes the following steps: Two-stage deep-drawing Two-stage stamping Folding to a hollow structure Joining of the overlapping sheet ends by MIG-welding.Version 2011 European Aluminium Association (auto@eaa.be)16
The axle must hold the wheels on the road in . (PRC) technology developed by Alcoa. The wall thickness of the cast subframe varies . In order to limit cost, but maintain light weight, the subframe was designed as a one-piece casting with a high degree of fun
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